when a few million years don’t mean much…

August 30, 2010

Oh those scientists with their constant corrections. Slightly more than a century ago, they said our planet and the entire solar system was a few hundred million years old, then they said it was 4.56 billion years old after fiddling around with radioactive isotopes in asteroids and meteors. Now, they’re changing the age of the solar system once again. How can we trust them after decades of jumping around and constant re-measuring? So how old is our solar system supposed to be now? About 4.5682 billion years? But wait, that’s a correction of only 300,000 to 2 million years from the date we have now. What’s the big deal? Well, the big deal is that this date solves a small controversy in planetary science and allows scientists to even further refine how to better measure the ages of astronomical objects based on radioactive decay, and how to prepare their samples.

While we can say with great certainty that our solar system is between 4.56 and 4.57 billion years old, getting to a more accurate number was a little tricky. Ordinarily, chemists measure the amounts of isotopes formed from the decay of more unstable elements. Since this decay happens at a very steady rate, you can compare the relative amounts of isotopes in a sample of a meteorite and come up with an accurate age. But there was a bit of a snag with dating the isotopes of lead, aluminum and magnesium. The latter two seem to be several million years older than the lead, which is a little odd to say the least. So a team of geochemists decided on a serious look into those troublesome lead isotopes, particularly 206Pb and 207Pb, formed by the decay of two isotopes of uranium. And they weren’t just going to take another measurement. They used another meteorite and washed it with a cocktail of acids to remove every last bit of contamination they could before testing.

Well, wouldn’t you know it, the results from the lead isotopes now match up with the older dates, showing that the readings of aluminum-26 and magnesium-26, were right, and that the other lead isotopes were probably contaminated with something that slightly offset their ages during measurements. Problem solved. But wait, you may ask, will we have another correction to the age of the solar system in the future? After all, if we’re now discussing one, maybe another technique will yield another estimate? And it very well could. However, I would hesitate to label this 300,000 to 1.9 million year refinement to a 4.568 billion year old system a correction and posit that it was in fact just a refinement necessary to understand how the infant solar system formed, but not very meaningful to those of us who are little more than informed laypersons on the subject. For us, the age of the planet or the solar system hasn’t actually changed at all in absolute terms, and we can just as confidently say that Earth is slightly over 4.5 billion years old. And continue to make fun of those who disagree, especially if they try to use the headlines about this refinement as evidence that “scientists keep changing their story.”

See: Bouvier, A., et al. (2010). The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion Nature Geoscience DOI: 10.1038/ngeo941